Inorganic powders, for example, zinc oxide or titanium oxide microparticles having a particle diameter of several tens to a hundred of nanometers, have been often used as an additive for sunscreen cosmetics, inks for exterior applications, food packaging materials, and the like because of their properties. When these powders are untreated, since the surfaces are hydrophilic, these powders may flow off by sweat or rain. Therefore, particularly in cosmetic applications, these powders are often used by subjecting the surfaces thereof to a water-repellent treatment with a silicone or a metallic soap.
On the other hand, since primary particles of inorganic powders generally tend to aggregate, the powders have been widely used by dispersing the powders in a dispersion medium, as described in Patent Document 1 or Patent Document 2, in order to exert sufficiently the visible light transparency and ultraviolet blocking performance of zinc oxide or titanium oxide microparticles. However, in conventional dispersions, water-repellent powders have been dispersed in oils, and hydrophilic powders have been dispersed in water, alcohol, glycol, or the like.
If an inorganic powder subjected to a water-repellent treatment can be dispersed in water, an effect by a powder subjected to a water-repellent treatment as described above can be obtained when a water-based composition is used for various applications. However, the attempt has been scarcely tried. In addition, if a water-repellent powder can be dispersed in water, dispersions can be used without caring laws and regulations and hazardousness because of free of any flammable organic solvent.
In the field of coating, it is known that a pigment dispersing resin is used when a pigment is mixed in a water-based coating (for example, Patent Document 3). However, in the pigment dispersed paste of Patent Document 3, the pigment is not subjected to a water-repellent treatment. Also, in Patent Document 3, since the inorganic powder as a raw material is used in powder form, and the powder and other components are mixed and dispersed with the pigment dispersing resin, the powder is not designed to prevent the aggregation between particles sufficiently, and there is a problem that the physical properties tend to change depending on the raw materials. Further, since various resins or pigments are added to the powder, the powder is deficient in applicability, and has a difficulty for use as a raw material.
Further, one method has been performed that a thin film of an inorganic powder is formed by using a dispersion of an inorganic powder to make a coating film, and subjecting the obtained thin film to a heat treatment or a plasma treatment. In the formation of a thin film, the method has an advantage that a thin film can be formed without the risk of ignition because use of any solvent is not required by using an aqueous dispersion of an inorganic powder.